Nickel-containing catalysts are widely used to hydrogenate aromatic compounds in various hydrocarbon feedstocks. Because of the sensitivity of nickel catalysts to poisoning by sulfur compounds commonly found in such feedstocks, the feedstocks are normally desulfurized to a considerable degree prior to being contacted with the nickel catalyst. Despite the desulfurization step, it is not uncommon for small amounts of sulfur impurities to remain in the feedstocks, including aromatic sulfur compounds, such as thiophene, benzothiophene and dibenzothiophene, which are particularly poisonous to supported nickel catalysts.
Because the poisoning of nickel catalysts by sulfur compounds is a severe world wide problem, extensive studies have been conducted in various laboratories in an attempt to determine the mechanism of sulfur poisoning, sometimes with conflicting results. For example, in the work by Poels, E. K., van Beek, W. P., den Hoed, W., Visser, C. (1995); Fuel Vol. 74 No. 12, pp 1800-1805, sulfur poisoning on a variety of nickel catalysts having a wide range of nickel surface area was evaluated. The authors concluded for all the catalysts tested that surface poisoning by sulfur was the predominate deactivation mechanism. This study suggested that sulfur absorption could be switched from surface to bulk using higher temperature and lower sulfur content in the feeds. However, they concluded that moving into bulk sulfur absorption did not extend catalyst life, as a surface layer still controlled catalyst deactivation. Others have reported that bulk sulfur absorption can occur with thiol type sulfur, but not with thiophenes. While there may be disagreement as to the precise mechanism of sulfur poisoning, it is generally accepted that the toxicity of sulfur compounds found in hydrocarbon feedstocks increases with the molecular weight and the complexity of the molecule, with thiopheneic compounds, such as thiophene, benzothiophene and dibenzothiophene being especially detrimental to nickel catalysts. A possible explanation for this is that higher molecular weight sulfur compounds, such as thiopheneic compounds, are not as readily decomposed as thiols, sulfides and mercaptans, but instead are adsorbed on the surface of the nickel catalyst forming a stable surface species which blocks active catalyst sites. This adsorption of thiopheneic compounds on the surface of the catalyst is generally believed to be irreversible due to the high heat of adsorption of these compounds. Since surface adsorption of sulfur compounds reduces active sites, catalyst vendors often quote catalyst lifetimes based on the sulfur in the feed and flows to get roughly one layer coverage of sulfur on the surface of the catalyst. Nickel based catalysts used to hydrogenate aromatics in feedstocks containing thiopheneic compounds generally have shorter catalyst lives than feedstocks containing lower molecular weight sulfur compounds, because of the tendency of the thiopheneic compounds under conventional process conditions to be adsorbed on the surface of the catalyst, thereby deactivating it. Accordingly, it can be seen that an aromatics hydrogenation process operated in such a manner that thiopheneic compounds in the feedstock did not poison or deactivate the nickel based catalyst employed in the process, would be highly desirable. The present invention provides such an improved process.